Different types of memory devices have been proposed in the last years. For example, a phase-change E2PROM is a non-volatile memory exploiting the properties of a material that can be reversibly switched between an amorphous phase and a crystalline phase. The phase-change material exhibits different electrical characteristics depending on its phase, each one representing a corresponding logic value. An example of a phase-change E2PROM is described in U.S. Pat. No. 5,166,758.
Typically, the memory device includes a matrix of memory cells, each one consisting of a functional element connected in series to an access element (such as the base-emitter junction of a bipolar transistor). The memory cells are arranged between corresponding word lines and bit lines.
In a stand-by condition, the word lines are kept at high voltage, so that all the access elements are reverse biased. However, a leakage current flows through each access element; for this reason, the bit lines are generally connected to a terminal providing a reference voltage, in order to avoid charging a stray capacitor associated with each bit line to a dangerous voltage.
During a writing or reading operation, all the bit lines are disconnected from the reference terminal. The voltage on a selected word line is then lowered, and one or more selected bit lines are driven to a voltage corresponding to the operation to be performed (while the other bit lines are left floating). In this condition, the access elements associated with the selected word line and the selected bit lines are forward biased.
A drawback of the solution described above is that the leakage currents in each deselected bit line charge the corresponding stray capacitor. The voltage reached on the deselected bit line can turn on the access element associated with the selected word line.
In this condition, the current flowing through the corresponding functional element raises the voltage on the selected word line; this voltage increase introduces a disturb in the operation of the memory device.
An additional drawback arises when the current flowing through the functional element reaches a value sufficient to switch the phase of the material. The phase switch can cause an undesired change of the value stored in the memory cell. In any case, even when the phase switch is only transient, a wrong value can be read if the memory cell is selected before a corresponding recovery time.
The problem is exacerbated when the temperature increases, since the leakage current typically depends on the temperature according to an exponential law. In addition, the total leakage current injected into each functional element in the selected word line is directly proportional to the number of access elements in the corresponding deselected bit line.